In many systems and devices, especially, for example, devices and systems which include high speed digital communication circuits, crosstalk interference between various electrical signals can be a significant problem—and a difficult one to understand and diagnose. Commonly, crosstalk interference may be produced by two parallel signal lines radiating energy onto each other. In general, herein we refer to the signal which generates the crosstalk interference as an “aggressor signal” (also known as an “interfering signal”) and the signal which experiences the crosstalk interference as a “victim signal.” Of course when two signals produce mutual crosstalk interference, each signal may be both an aggressor signal in one case, and a victim signal in the other case.
Power supplies can also create crosstalk interference to signals. In many cases, power supply crosstalk interference onto signals can be just as important or more important to understand and diagnose as crosstalk interference between two signal lines. One reason for this is that different mechanisms and affects may pertain to power supply crosstalk as compared to crosstalk interference between two signal lines. For example, in general crosstalk interference between signal lines adds a voltage error to the original victim signal. However, in contrast, power supply crosstalk may not only add a voltage error to the victim, it may also amplitude modulate and phase modulate the victim signal. It should be noted that some aggressor signals other than a supply voltage may produce time or phase modulation of a victim signal (e.g., simple linear crosstalk between two signals after propagating through a subsequent voltage limiting buffer amplifier). Adding voltage error and/or phase modulation can affect the timing of edges or bit transitions in the victim signal's waveform, resulting in jitter. When the aggressor signal is a supply voltage, this may be referred to as power supply induced jitter (PSIJ). However, the phase modulation mechanism is particularly difficult to analyze because it is a time-variant process that does not lend itself to other linear stationary analysis techniques. Phase-modulated PSIJ may occur, for example, when a power supply provides a supply voltage to an oscillator (e.g., voltage controlled oscillator (VCO) and/or a phase-locked loop (PLL) which is used to generate a clock signal for a circuit, which in turn is used to determine the timing of the edges or bit transitions of a victim signal. In that case, power supply noise on the supply voltage may produce jitter in the clock signal, which in turn may produce PSIJ in the victim signal.
The analysis and diagnosis of crosstalk induced jitter such as PSIJ in a given device may be difficult and complicated.
For example, circuit simulation may be employed to analyze and diagnose PSIJ for one or more signal lines of a particular device. Given a circuit model of the particular device, software simulation tools may be employed to estimate the amount of PSIJ for a given signal line.
However, such circuit simulation has drawbacks. For one thing, the simulation results will only be as good as the circuit model. Producing accurate circuit models can be difficult and time-consuming as many circuits are complicated and have a large number of components. And if an accurate circuit model is produced, every time that a change is made to the device which is being analyzed, the circuit model must be updated. Furthermore, running the simulations can also require a lot of effort and can be time consuming. Moreover, it can be very difficult to produce an accurate simulation since in many cases the PSIJ may be created or affected by non-linearities and parasitic impedances in the circuit, which—unlike nominal circuit values—are typically not known in advance and may be difficult to ascertain. Finally, because of this, PSIJ performance may vary significantly from individual device to individual device even when the devices are designed to be identical. So accuracy of PSIJ estimates produced from circuit modeling and simulation is an issue.
One improvement for analyzing and diagnosing PSIJ for a device is to use actual measurements of a sample of the device to construct a circuit model, rather than constructing the circuit model from circuit diagrams or schematics. For example, to construct the circuit model one may disconnect the power supply from the rest of the device under test and replace it with an external supply which can be controlled to artificially generate a range of disturbance(s), and then measure the corresponding effect on the signal line(s) of interest as a function of the disturbance(s) across an expected range of interest. While this approach may potentially yield more accurate results, it can be tedious, invasive, time consuming, and require a lot of very expensive equipment.
It would be desirable to provide another technique for analyzing and diagnosing crosstalk induced jitter in a device under test.